Bacterial DNA has immune stimulatory effects to activate B cells and natural killer cells, but vertebrate DNA does not. Tokunaga T et al. (1988) Jpn J Cancer Res 79:682-6; Tokunaga T et al. (1984) JNCI 72:955-62; Messina J P et al. (1991) J Immunol 147:1759-64; and reviewed in Krieg, 1998, In: Applied Oligonucleotide Technology, C. A. Stein and A. M. Krieg, (Eds.), John Wiley and Sons, Inc., New York, N.Y., pp. 431-448) and Krieg A M (2002) Annu Rev Immunol 20:709-60. It is now understood that these immune stimulatory effects of bacterial DNA are a result of the presence of unmethylated CpG dinucleotides in particular base contexts (CpG motifs), which are common in bacterial DNA, but methylated and underrepresented in vertebrate DNA. Krieg A M et al. (1995) Nature 374:546-9; Krieg A M (1999) Biochim Biophys Acta 1489:107-16.
The immune stimulatory effects of bacterial DNA can be mimicked with synthetic oligodeoxynucleotides (ODN) containing these CpG motifs. Such CpG ODN have highly stimulatory effects on human and murine leukocytes, inducing B-cell proliferation; cytokine and immunoglobulin secretion; natural killer (NK) cell lytic activity and interferon gamma (IFN-γ) secretion; and activation of dendritic cells (DCs) and other antigen-presenting cells to express costimulatory molecules and secrete cytokines, especially the Th1-like cytokines that are important in promoting the development of Th1-like T-cell responses. These immune stimulatory effects of native phosphodiester backbone CpG ODN are highly CpG-specific in that the effects are dramatically reduced if the CpG motif is methylated, changed to a GpC, or otherwise eliminated or altered. Krieg A M et al. (1995) Nature 374:546-9; Hartmann G et al. (1999) Proc Natl. Acad Sci USA 96:9305-10.
In early studies, it was thought that the immune stimulatory CpG motif followed the formula purine-purine-CpG-pyrimidine-pyrimidine. Krieg A M et al. (1995) Nature 374:546-9; Pisetsky D S (1996) J Immunol 156:421-3; Hacker H et al. (1998) EMBO J 17:6230-40; Lipford G B et al. (1998) Trends Microbiol 6:496-500. However, it is now clear that mouse lymphocytes respond quite well to phosphodiester CpG motifs that do not follow this “formula” (Yi A K et al. (1998) J lmmunol 160:5898-906), and the same is true of human B cells and dendritic cells (Hartmann G et al. (1999) Proc Natl Acad Sci USA 96:9305-10; Liang H et al. (1996) J Clin Invest 98:1119-29).